Directly reducing iron oxide in a rotary kiln with pellitized carbonaceous material

ABSTRACT

IN A PROCESS OF DIRECTLY REDUCING AN IRON ORE IN ADMIXTURE WITH A CARBONACEOUS MATERIAL IN A ROTARY KILN, AT LEAST THAT PORTION OF THE UNCONSUMED CARBONACEOUS MATERIAL REMOVED FROM THE KILN PRODUCT HAVING A PARTICLE SIZE BELOW 1 MILLIMETER IS PELLETIZED, PREFERABLY WITH THE ADDITION OF A HYDROCARBONACEOUS BINDER, AND RECYCLED.

United States Patent Office 3,684,483 Patented Aug. 15, 1972 DIRECTLYREDUCING IRON OXIDE IN A ROTARY KILN WITH PELLITIZED CARBO- NACEOUSMATERIAL Gerhard Renter, Frankfurt, and Dietrich Werner, Offenthal,Germany, assignors to Metallgesellschaft Aktiengesellschaft, Frankfurtam Main, Germany No Drawing. Filed Jan. 14, 1970, Ser. No. 2,949 Claimspriority, application Germany, Feb. 28, 1969, P 19 10 147.4 Int. Cl.C21b 11/06, 13/08 U.S. Cl. 75-33 Claims ABSTRACT OF THE DISCLOSURE In aprocess of directly reducing an iron ore in admixture with acar-bonaceous material in a rotary kiln, at least that portion of theunconsumed carbonaceous material removed from the kiln product andhaving a particle size below 1 millimeter is pelletized, preferably withthe addition of a hydrocarbonaceous binder, and recycled.

PREAMBLE It is known to directly reduce iron oxide-containing materialsin a rotary kiln in admixture with solid carbonaceous materials.Unconsumed carbonaceous material recovered from the kiln product isusually recycled.

The solid carbonaceous materials serve as reducing agents and produceall or part of the heat which is required in the process. If only partof the heat requirement is met by the combustion of the carbonaceousmaterials, the remaining quantity of heat is supplied to the processwith the aid of burners.

To ensure a substantial metallization and a protection of the resultingsponge iron against re-oxidation, the matter discharged from the kilnmust contain a certain amount of surplus or unconsumed carbonaceousmaterial. Unless the entire matter discharged from the kiln is directlysubjected to further processing, e.g., in an electric furnace, thisunconsumed carbonaceous material must be separated from the matter whichhas been discharged from the kiln.

For economic reasons, the surplus fuel which has been separated must bere-used. If the surplus fuel has a particle size in excess of about 1millimeter, it can be recycled to the process without difficulty and canbe charged into the rotary kiln at the charging end, together with thecharge, or at a subsequent point further in the kiln.

It is difiicult, however, to recycle fuel having a particle size belowabout 1 millimeter in this manner because such fuel would be entrainedin the rotary kiln and a considerable part of such fuel would be carriedout by the exhaust gases leaving the kiln.

In the previously known processes, considerable losses were caused bythe fact that the exhaust gases contained solid carbonaceous material.Also any subsequent treatment or use of the exhaust gases would beplagued by the entrained solids. For this reason, the previously usedcarbonaceous materials were selected so that the resulting amount ofunconsumed carbonaceous material having a particle size below about 1millimeter in the matter discharged from the kiln was as small aspossible.

While the surplus carbonaceous fines in the matter discharged from thekiln can be utilized by burning said fines in separate furnaces, thismeasure is not economical in many cases.

THIS INVENTION This invention provides a process of directly reducingiron oxide-containing materials in a rotary kiln wherein the unconsumedcarbonaceous fines contained in the matter discharged from the kiln andhaving a particle size below about 1 millimeter are recycled to therotary kiln and any entraining of the recirculated carbonaceous materialby the exhaust gases is substantially eliminated. It is possible,according to this invention, to process solid carbonaceous materialwhich has disintegrated in large part in the rotary kiln to a particlesize of less then 1 millimeter.

According to the invention, the surplus carbonaceous material which hasbeen separated from the matter discharged from the kiln is treated toremove part of the ash therefrom and is then pelletized by a rollingoperation with the addition of water and/or binders, and only theresulting pellets are recycled and charged to the rotary kiln.

Part of the ash which has been produced from the carbonaceous materialmust be removed before the material is pelletized because the ashcontent would otherwise continuously increase. It is suitable to removethat fraction which has the highest ash content from the surplus.

The surplus carbonaceous material is generally not directly available inthe particle size which is required for a pelletization so that thematerial should be comminuted. The surplus carbonaceous material ispreferably comminuted to the particle size which is required for thepelletization after part of the ash has been removed because in thiscase the costs of comminuting the amount of ash which has been removedis eliminated.

If the surplus carbonaceous material has to be present in a smallparticle size before the ash can be removed, the entire surplus iscomminuted to the particle size which is required for the pelletizationand then part of the ash is separated before the pelletization iseffected.

In a preferred embodiment of this invention, only that surpluscarbonaceous fuel which has a particle size below about 1 millimeter andis contained in the matter discharged from the kiln is pelletized. Thesurplus having a particle size above about 1 millimeter canberecirculated to the kiln without being pelletized becuse the losses ofthat particle size fraction caused by the exhaust gases are much smallerthan with the fine fraction.

In a further embodiment of this invention, the surplus carbonaceous fuelto be pelletized in comminuted so that 50-80 percent, preferably about60 percent thereof, has a particle size below 0.04-0.06 millimeter,preferably below 0.05 millimeter. The required state of fineness willdepend on the desired strength of the pellets and on the properties ofthe carbonaceous material. In an embodiment of the invention, thepellets are produced in a size up to about 20 millimeters, preferably8-12 millimeters.

Water in an amount up to about 40 percent by weight is preferably addedin the production of the pellets.

The binders which can be added to the pellets consist preferably ofheavy organic binders, such as pitch and tar, which have preferably beenrecovered from the same carbonaceous material. These binders are addedin an amount up to 15 percent by weight. The binder ma'y alternativelyconsist, e.g., of inorganic materials, such as bentonite in an amount upto 3 percent by Weight, or boiler ash in an amount up to 10 percent byweight. A mixture of these binders can also be used.

The strength of the pellets will be sufiicient if they do notdisintegrate while travelling through the first 2-4 meters of the rotarykiln into which they are charged. Alternatively, the pellets can be madein such a strength that they can travel along the entire length of thekiln substantially without disintegration. These strengths can bereadily achieved without a thermal hardening process.

EXAMPLE A rotary kiln was charged with fresh coal in the form ofRheinische lignite having the following composition:

Percent Water Ash 4.8 fixed 45.5 Volatiles 39.5

Net calorific value 5,150 kcal./ kg.

The matter discharged from the rotary kiln contained surplus carbon inan amount of 10 percent by weight based on the charge ore. Thislow-temperature coke contained 83.7 percent fixed carbon. The matterdischarged from the furnace was subjected to electrostatic magneticseparation whereby 167 kilograms of non-magnetic material were separatedper metric ton of sponge iron.

The separated non-magnetic material was separated into a fraction below1 millimeter, amounting to 50 kilograms, and a fraction above 1millimeter, amounting to 117 kilograms, 2 kilograms were separated fromthe fraction below 1 millimeter and discarded The remainder of 48kilograms was ground in a mill until 60 percent of it had a particlesize below 0.04 millimeter. It was then mixed with 3.5 kilograms pitchand 12.5 kilograms water and pelletized on a pelletizing plate to formpellets having an average diameter of 10 millimeters. 5 kilograms offixed carbon were lost during the grinding, mixing and pelletizingoperations so that the 5-8 kilograms of green pellets produced contained35 kilograms of fixed carbon from the recirculated carbon. These pelletsand the fraction above 1 millimeter were charged into the charging endof the rotary kiln. The entire recirculated coke contained 133 kilogramsof fixed carbon.

The saving compared to a recirculation of the surplus carbon without thepelletizing according to the invention amounted to 26 kilograms of fixedcarbon, corresponding to 90 kilograms fresh coal. The heat consumptionwas reduced from 3,960,000 to 3,600,000 kcal. per metric ton of spongeiron.

In another test, the entire fraction below 1 millimeter was ground.Thereafter, 2 kilograms of this fraction were separated and discarded.The results were virtually the same.

The process according to the invention has the advantage that solidcarbonaceous materials can be used which disintegrate to a large extentin the kiln and that the surplus in the matter discharged from thefurnace can be recirculated to the rotary kiln so that an economicbenefit is obtained. Solid carbonaceous materials which disintegrate toa large extent in the kiln are, e.g., lignite or high-ash coal. Aspecial advantage resides in that these materials can even be charged atthe charging end of a rotary kiln to which the charge is supplied at ahigh temperature. In such process, the velocity of the exhaust gases atthe end where the charge is received is particularl'y high andfine-grained materials are entrained by the exhaust gases to a largeextent.

We claim:

1. In a process of directly reducing an iron oxidecontaining materialcharged into one end of a rotary kiln in admixture with a solidcarbonaceous material as a reducing agent wherein unconsumedcarbonaceous material recovered from the kiln product is recycled, theimprovement comprising: separating said unconsumed carbonaceous materialfrom said kiln product, treating the material so recovered to remove atleast a substantial part of the ash therefrom, separating from thebalance of the resulting material that portion having a particle sizeabove about 1 millimeter and directly recycling it to the kiln,pelletizing the balance of the resulting material having a particle sizebelowabout l millimeter into pellets having a particle size in the rangeof 8 to 20 millimeters and charging the resulting pellets into saidrotary kiln at the charging end thereof.

2. The process of claim 1 wherein said resulting material is comminutedprior to pelletizing.

3. The process of claim 2 wherein said resulting material is comminutedso that in the range of 50 to 80 percent thereof has a particle sizebelow about 0.06 millimeter.

4. The process of claim 1 wherein the pellets are made with an additionof water amounting up to about 40 percent by weight.

5. The process of claim 1 wherein the pellets are made with an additionof up to about 15 weight percent of a heavy hydrocarbonaceous binder.

References Cited UNITED STATES PATENTS 3,235,375 2/ 1966 Meyer et al.7'5-36 X 3,425,824 2/1969 Heitmann -33 3,469,970 9/1969 Heitmann 75-332,834,044 5/1958 Antonson et al. 23-314 UX 3,294,567 12/ 1966 Dobbin etal. 23-314 X 3,391,234 7/ 196 8 Walenciak et al. 23-314 X 2,880,0833/1959 Wienert 75-33 3,097,090 7/1963 Aamot 75-33 3,180,725 4/ 1965Meyer et al. 75-33 3,462,262 8/1969 Koch 75-33 2,687,290 8/ 1 954Garoutte et a1. 75-25 'UX 3,326,669 6/ 196-7 Stirling 75-25 X HEN-RY W.TAR-RING 11, Primary Examiner US. Cl. X.R. 23-314

